Simpson coefficients of similarity between Laurentia and Avalonian euconodont shelf faunas show a progressive decrease in endemism through the Late Ordovician to Early Silurian, consistent with the northward drift of Avalonia toward Laurentia. As Avalonia drifted into warmer tropical latitudes in the Ashgillian, there was a marked increase in faunal similarity; this correlates with the southward insinuation of Laurentian taxa into Avalonia, followed by extinction of Avalonian endemic species. Euconodonts and many other shelly invertebrate groups were able to cross the Iapetus Ocean by the Middle Ordovician. By the end of the middle Llandovery, euconodont faunas were identical on the Laurentian and Avalonia shelves, indicating the free exchange of species within the tropics. Our results are consistent with paleomagnetic data indicating that the Iapetus Ocean was closed by the Middle Silurian. The spread of cosmopolitan species and extinction of endemic species, allied to plate movement into new climate belts, may be a significant mechanism in the reduction of gamma diversity.